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                                                                     Exhibit 4.5

                   EXCHANGE AND REGISTRATION RIGHTS AGREEMENT

         THIS AGREEMENT (the "Agreement") is made as of August 6, 1996 by and
among BBN CORPORATION, a Massachusetts corporation (the "Company"), BBN PLANET
CORPORATION, a Massachusetts corporation ("Planet"), and AT&T VENTURE COMPANY,
L.P., a Delaware limited partnership ("AT&T" or the "Holder").

         WHEREAS, the Company owns 23,800,000 of the issued and outstanding
shares of Planet common stock, The Board of Trustees of the Leland Stanford
Junior University owns 200,000 of the issued and outstanding shares of Planet
common stock, Harvard University owns 10,000 of the issued and outstanding
shares of Planet common stock, Boston University owns 10,000 of the issued and
outstanding shares of Planet common stock, and Massachusetts Institute of
Technology owns 10,000 of the issued and outstanding shares of Planet common
stock, together constituting all of the issued and outstanding shares of Planet
common stock;

         WHEREAS, the Holder owns 1,000,000 of the issued and outstanding shares
of Planet Series A Convertible Preferred Stock, $.01 par value, constituting all
of the issued and outstanding shares of Planet preferred stock and together with
the Planet common stock, constituting all of the capital stock of Planet; and

         WHEREAS, the Company wishes to acquire and the Holder wishes to
transfer all of the issued and outstanding shares of Planet's Series A
Convertible Preferred Stock, $.01 par value in a transaction intended to be
treated as a taxable transaction and not as a reorganization within the meaning
of IRC Section 368(a), as amended.

         NOW, THEREFORE, in consideration of the mutual covenants and agreements
herein contained, the parties to this Agreement agree as follows:

         1. Exchange of the Series A Preferred Stock. Subject to the terms and
conditions hereof, the Company agrees to assign, transfer, and deliver to AT&T
(i) 400,000 shares (the "Shares") of the Company's Common Stock, $1.00 par value
(the "Common Stock"), (ii) the Put Option (as defined in Section 6 hereof), and
(iii) $1,000 in cash, all in exchange for, and AT&T agrees to accept such
consideration in complete exchange for and to transfer to the Company 1,000,000
shares of Planet's Series A Convertible Preferred Stock, $.01 par value (the
"Series A Preferred Stock") and for the cancellation of the rights of AT&T
relating to such shares of Series A Preferred Stock under the agreements
canceled hereunder as provided in Section 7.3 of this Agreement. This exchange
of Series A Preferred Stock for Common 
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Stock shall occur at a closing (the "Closing") to be held at the offices of
Ropes & Gray, Boston, Massachusetts on the date of this Agreement (the "Closing
Date") at 5:00 p.m.

         At the Closing, AT&T will deliver to the Company certificate(s)
representing the Series A Preferred Stock, duly endorsed in blank or accompanied
by separate stock powers, in each case in proper form for transfer and with
signature guaranteed, with any necessary stock transfer tax stamps affixed. At
the Closing, the Company will deliver to AT&T, in exchange, certificate(s)
representing the Shares, duly registered in the name of AT&T.

         2.  Registration Rights.

         (a) Demand Registration Rights. If following October 1, 1996 and prior
to the first date that all of the Shares may be sold immediately in accordance
with the provisions of Rule 144 promulgated under the Securities Act of 1933
(the "Demand Period"), Holder requests the Company to file a registration
statement under the Securities Act of 1933 (hereinafter, the "Securities Act")
for a public offering of Registrable Securities (as defined in Section 2(c)
hereof) equal to at least the lesser of (i) 50% of the Shares, or (ii) the
number of Registrable Securities the proposed aggregate offering price of which
is at least $3,000,000, the Company shall use its best efforts to register under
the Securities Act such Registrable Securities so requested. Notwithstanding the
foregoing, (i) the Company shall not be obligated to effect the filing of a
registration statement pursuant to this Section 2(a) during the 180 days
following the effective date of a registration statement pertaining to a public
offering of securities for the account of the Company, and (ii) if the Company
shall furnish to Holder requesting such registration a certificate signed by the
President of the Company stating that, in the good faith judgment of the Board
of Directors of the Company, it would not be in the best interests of the
Company and its stockholders generally for such registration statement to be
filed, the Company shall have the right to defer such filing for a period of not
more than 120 days after receipt of the request of Holder; provided, however,
that the Company may not utilize the right set forth in this subsection (ii) for
more than one (1) such 120-day period with respect to each such request. The
Company is obligated to effect no more than two (2) registrations pursuant to
this Section 2(a), provided that no such registration which shall not have
become and remained effective for a period of at least 120 days (or such shorter
period during which Holder shall have sold all Registrable Securities which were
so registered) shall be deemed to be a registration for any purpose of this
Section 2(a).

         (b) Piggyback Registration Rights. Whenever the Company proposes to
register any Common Stock for its own or others' account under the Securities
Act for a public offering for cash, other than a registration relating to
employee benefit plans, the Company shall give Holder, if holding Registrable
Securities, prompt written notice of its intent to do so. Upon the written
request of Holder given within fifteen days of such notice, the Company will use
its best efforts to cause to be included in such registration all of the
Registrable Securities which Holder requests. If the Company is advised in
writing in good faith by any managing underwriter of the securities being
offered pursuant to any registration statement

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under this Section 2(b) that the number of shares to be sold pursuant to such
registration statement is greater than the number of such shares which can be
offered without adversely affecting the offering, the Company shall first reduce
pro rata the number of shares offered for the accounts of holders of the
Company's Common Stock that do not hold registration rights similar to the
rights granted to Holder pursuant to this Section 2(b), and then may reduce pro
rata the number of shares offered for the account of Holder and for the accounts
of holders of the Company's Common Stock that have registration rights similar
to the rights granted to Holder pursuant to this Section 2(b) (based upon the
number of shares of Common Stock proposed to be sold pursuant to such
registration statement by Holder and such other holders of the Company's Common
Stock) to a number deemed satisfactory by such managing underwriter. In the
event of such a limitation, shares of persons not having registration rights
will not be included in the registration unless all Registrable Securities
requested to be included in the registration have been included. No piggyback
registration right in this Section 2(b) shall be construed to limit the demand
registration right contained in Section 2(a) hereof. The Company shall not after
the date hereof grant piggyback registration rights to any third party which
provide such third party seniority over Holder with respect to the inclusion of
securities held by such third party over Registrable Securities under this
Section 2(b), provided that the foregoing provisions of this sentence shall not
apply to a registration statement filed by the Company pursuant to the exercise
by such third party of "demand" registration rights.

         (c) Definition of Registrable Securities. "Registrable Securities"
means (i) the Shares and (ii) shares of Common Stock or other securities
convertible into Common Stock received as a stock dividend or other distribution
in respect of the Shares; provided, however, that Registrable Securities shall
not include any such shares of Common Stock that as of the date of the
determination may be sold without limitation pursuant to Rule 144(k) under the
Securities Act.

         (d) Registration Procedures. All expenses incurred in connection with
the registrations under this Section 2 (including all registration, filing,
qualification, printer's, and accounting fees and the reasonable fees of one
counsel for the holders, but excluding underwriting commissions and discounts)
shall be borne by the Company. In connection with registrations under this
Section 2, the Company shall (i) use its best efforts to prepare and file with
the Commission as soon as reasonably practicable, a registration statement with
respect to the subject Registrable Securities and use its best efforts to cause
such registration to promptly become and remain effective for a period of at
least 120 days (or such shorter period during which Holder shall have sold all
Registrable Securities which were so registered); (ii) use its best efforts to
register and qualify the Registrable Securities covered by such registration
statement under applicable state securities laws as Holder shall reasonably
request for the distribution of the Registrable Securities; and (iii) take such
other actions as are reasonable and necessary to comply with the requirements of
the Securities Act and the regulations thereunder, or the reasonable request of
Holder, with respect to the registration and distribution of the Registrable
Securities. The Company is not obligated to effect registration or qualification
under this Section 2 in any jurisdiction requiring it to qualify to do business

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(unless the Company is otherwise required to be so qualified) or to execute a
general consent to service of process.

         (e) Underwriting Arrangement. In connection with each registration
pursuant to Sections 2(a) and (b) above covering an underwritten public
offering, the Company and Holder agree to enter into a written agreement with
the managing underwriter in such form and containing such provisions as are
customary in the securities business for such an arrangement between such
underwriter and companies of the Company's size and investment stature;
provided, that Holder shall not be required to agree to any provision making
Holder jointly liable or jointly and severally liable with any other party
including the Company.

         (f) Notification. The Company shall promptly notify Holder if
participating in a registration statement covering Registrable Securities of any
event which results in the prospectus included in such registration statement,
as then in effect, containing an untrue statement of a material fact or omitting
to state a material fact required to be stated therein or necessary to make the
statements therein not misleading in the light of the circumstances then
existing. The Company shall use its best efforts to make appropriate changes to
such prospectus and provide copies of such revised prospectus to Holder.

         (g) Furnishing of Documents. At the request of Holder, if participating
in a registration statement, the Company will furnish to each underwriter, if
any, and Holder, a legal opinion of its counsel and a letter from its
independent certified public accountants, each in customary form and substance,
at such time or times as such documents are customarily provided in the type of
offering involved.

         (h) Prospectuses. From time to time the Company shall amend or
supplement any registration statement pursuant to which any of the Registrable
Securities are being offered pursuant to a registration under this Agreement and
the prospectus contained therein to the extent necessary to comply with the
Securities Act and any applicable state securities or Blue Sky laws. The Company
shall also provide Holder with as many copies of the prospectus contained in any
such registration statement in which Registrable Securities are included as it
may reasonably request.

         (i) Indemnification by the Company. The Company will indemnify and hold
harmless Holder and each underwriter of the Registrable Securities being sold by
Holder, and each controlling person of Holder and underwriter, against all
claims, losses, damages, and liabilities (or actions in respect thereof) arising
out of or based on any untrue statement (or alleged untrue statement) of a
material fact contained in any registration statement relating to such
Registrable Securities (or in any related registration statement, notification,
or the like) or any omission (or alleged omission) to state therein a material
fact required to be stated therein or necessary to make the statements therein
not misleading, or any violation by the Company of any rule or regulation
promulgated under the Securities Act applicable to the Company and relating to
action or inaction required of the Company in connection with any such

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registration, qualification, or compliance, and will reimburse Holder and each
such underwriter and controlling person for any legal or any other expenses
reasonably incurred in connection with investigating or defending any such
claim, loss, damage, liability, or action and will enter into an indemnification
agreement with Holder and such underwriter containing customary provisions,
including provisions for contribution, as Holder or any underwriter shall
reasonably request; provided, however, that the Company will not be liable in
any such case to the extent that any such claim, loss, damage, or liability
arises out of or is based on any untrue statement or omission based upon written
information furnished to the Company by Holder or underwriter and stated to be
specifically for use therein.

         (j) Indemnification by Holder. Holder, if so participating in a
registration statement, will indemnify and hold harmless the Company, each of
its directors, each of its officers who has signed the registration statement
and each person, if any, who controls the Company within the meaning of Section 
15 of the Securities Act, against all claims, losses, damages and liabilities
(or actions in respect thereof) arising out of or based on any untrue statement
(or alleged untrue statement) of a material fact contained in any registration
statement relating to the Registrable Securities (or in any related registration
statement, notification or the like) or any omission (or alleged omission) to
state therein a material fact required to be stated therein or necessary to make
the statements therein not misleading, and will reimburse the Company and each
such director, officer or controlling person for any legal or any other expenses
reasonably incurred in connection with investigating or defending any such
claim, loss, damage, liability, or action and will enter into an indemnification
agreement with the Company containing customary provisions, including provisions
for contribution, as the Company or each such person shall reasonably request;
provided, however, that Holder will not be liable in any such case except to the
extent that any such claim, loss, damage, or liability arises out of or is based
on any untrue statement or omission based upon written information furnished to
the Company by Holder and stated to be specifically for use therein.
Notwithstanding any of the foregoing to the contrary, in no event shall the
liability of Holder for indemnification under this Section 2(j) exceed the
lesser of (i) that percentage of the total amount of such losses, claims,
damages or liabilities indemnified against which equals the percentage obtained
by dividing the total number of Registrable Securities sold by Holder pursuant
to the registration statement by the total number of securities of the Company
sold pursuant to the registration statement, and (ii) the net proceeds received
by Holder in the offering.

         (k)      Indemnification Procedures.

                  (i) Promptly after receipt by any person entitled to
indemnification under Sections 2(i) or 2(j) (and "Indemnified Party") of notice
of the commencement of any action in respect of which indemnity may be sought
against any person under Sections 2(i) or 2(j) (and "Indemnifying Party"), such
Indemnified Party shall notify all Indemnifying Parties in writing of the
commencement thereof (provided, however, that failure to so notify an
Indemnifying Party shall not relieve any Indemnifying Party from any liability
it may have hereunder except

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to the extent that the Indemnifying Party who did not receive such notice shall
have been materially prejudiced by such failure) and, subject to the provisions
hereinafter stated, the Indemnifying Party shall be entitled to assume the
defense of such action (including the employment of counsel, who shall be
counsel reasonably satisfactory to such Indemnified Party), and the payment of
expenses insofar as such action shall relate to any alleged liability in respect
of which indemnity may be sought against the Indemnifying Party.

                  (ii) The Indemnified Party shall have the right to employ
separate counsel and assume its own legal defense in any such action and to
participate in the defense thereof, but the fees and expenses of such counsel
subsequent to any assumption of the defense by the Indemnifying Party shall not
be at the expense of the Indemnifying Party unless the employment of such
counsel has been specifically authorized in writing by the Indemnifying Party.
The Indemnifying Party shall not be liable to indemnify any Indemnified Party
for any settlement of any such action effected without the Indemnifying Party's
written consent. The Company shall not, except with the approval of each party
being indemnified under this Agreement, consent to entry of any judgment or
enter into any settlement which does not include as an unconditional term
thereof the giving by the claimant or plaintiff to the parties being so
indemnified of a release from all liability in respect to such claim or
litigation.

         (l) Contribution. In order to provide for just and equitable
contribution to joint liability under the Securities Act in any case in which
any Indemnified Party exercising rights under this Agreement, or any controlling
person of any such Indemnified Party, makes a claim for indemnification pursuant
to Section 2(i) or 2(j) but it is judicially determined (by the entry of a final
judgment or decree by a court of competent jurisdiction and the expiration of
time to appeal or the denial of the last right of appeal) that such
indemnification may not be enforced in such case notwithstanding the fact that
this Agreement provides for indemnification in such case, then the Indemnifying
Party and such Indemnified Party will contribute to the aggregate losses,
claims, damages or liabilities to which they may be subject (after contribution
from others) in such proportion as is appropriate to reflect the relative fault
of the Indemnifying Party on the one hand and of the Indemnified Party on the
other in connection with the statements or omissions which resulted in such
losses, claims, damages or liabilities, as well as any other relevant equitable
considerations. The relative fault of the Indemnifying Party on the one hand and
of the Indemnified Party on the other shall be determined by reference to, among
other things, whether the untrue or alleged untrue statement of a material fact
or omission or alleged omission to state a material fact relates to information
supplied by the Indemnifying Party on the one hand or by the Indemnified Party
on the other, and each party's relative intent, knowledge, access to information
and opportunity to correct or prevent such statement or omission; provided,
however, that, in any such case, no person guilty of fraudulent
misrepresentation (within the meaning of Section 11(f) of the Securities Act)
will be entitled to contribution from any person who was not guilty of such
fraudulent misrepresentation.

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         (m) Exchange Act Registration. As long as the Company is subject to the
reporting requirements of either Section 13 or Section 15(d) of the Securities
Exchange Act of 1934 (the "Exchange Act"), the Company will use its reasonable
efforts to timely file with the Commission such information as the Commission
may require under either Section 13 or Section 15(d) of the Exchange Act; and in
such event, the Company shall use its reasonable efforts to take all action as
may be required as a condition of the availability of Rule 144 under the
Securities Act (or any successor exemptive rule hereafter in effect) with
respect to the Registrable Securities. The Company shall furnish to Holder
forthwith upon request (i) a written statement by the Company as to the
compliance with the reporting requirements of Rule 144, (ii) a copy of the most
recent annual or quarterly report of the Company as filed with the Commission,
and (iii) such other reports and documents as the Holder may reasonably request
in availing itself of any rule or regulation of the Commission allowing the
Holder to sell any such Registrable Securities pursuant to Rule 144 without
registration. The Company shall use its reasonable efforts to facilitate and
expedite transfers of the Registrable Securities pursuant to Rule 144 under the
Securities Act, which efforts shall include prompt notice to its transfer agent
to expedite such transfers of Registrable Securities.

         (n) Transfer or Assignment of Registration Rights. The rights to cause
the Company to register securities issued to Holder under this Section 2 may be
transferred or assigned by Holder (i) to AT&T Corp. or to any affiliate of AT&T
Corp., or (ii) to any other entity with the prior written consent of the Company
(which consent shall be given in the sole discretion of the Company). In either
case, Holder shall give written notice at the time of or within a reasonable
time after said permitted transfer or assignment, stating the name and address
of said transferee or assignee and identifying the securities with respect to
which such registration rights are being transferred or assigned, and the
transferee or assignee of such rights shall agree in writing to be bound by the
obligations of a holder of Registerable Securities under this Section 2.

         3. Purchase for Investment. AT&T acknowledges that the Shares will be
sold and exchanged without registration under the Securities Act on the basis
that the transactions contemplated by this Agreement do not involve any public
offering of securities. AT&T hereby confirms to the Company that it is acquiring
such securities for its own account, for investment only and without a view
toward the distribution thereof. AT&T hereby confirms that it is able to bear
the economic risk of an investment in the Shares acquired by it pursuant to this
Agreement and can afford to sustain a total loss on such investment and has such
knowledge and experience in financial and business matters that it is capable of
evaluating the merits and risks of the proposed investment and therefore has the
capacity to protect its own interests in connection with the purchase of the
Shares. AT&T acknowledges that it has been fully informed of the limitations
that are implicit in the fact that the Shares are not registered under the
Securities Act for sale to it, and may not be transferred absent registration or
an exemption therefrom, that the Company has no obligation to register any of
the Shares sold to AT&T other than as specifically provided in this Agreement,
and that the exemptions from the

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registration requirements of the Securities Act are limited, and agrees that
such securities shall be legended to note such restrictions on their transfer in
substantially the following form:

                           "THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN
                  REGISTERED UNDER THE SECURITIES ACT OF 1933 AND MAY NOT BE
                  OFFERED OR SOLD ABSENT AN OPINION OF COUNSEL REASONABLY
                  SATISFACTORY TO THE ISSUER THEREOF THAT SUCH REGISTRATION HAS
                  BEEN EFFECTED OR AN EXEMPTION THEREFROM IS AVAILABLE. TRANSFER
                  MAY BE REFUSED IN THE ABSENCE OF SUCH OPINION."

         AT&T acknowledges receipt of financial and other information concerning
the Company (including copies of its annual report to shareholders for fiscal
year 1995, its 10-K filing for the year ended June 30, 1995, its 10-Q filings
for the periods ended September 30, 1995, December 31, 1995, and March 31, 1996
and its proxy statement for the 1995 annual meeting), and that it has had the
opportunity to obtain additional information concerning the Company. AT&T
represents and agrees that it has not taken prior to the date of this Agreement,
and will not take after the date of this Agreement, any action that would make
unavailable the exemption provided in Section 4(2) of the Securities Act with
respect to the receipt by it of the Shares.

         Neither AT&T nor the Company has relied upon any agreement,
representation, or warranty of the other which is not contained in this
Agreement or in the schedules hereto (including the publicly filed documents of
the Company listed in the second paragraph of this Section 3), in connection
with the consummation of the transactions contemplated hereby.

         4. Representation and Warranties of AT&T. As of the Closing, AT&T
represents and warrants to the Company and Planet that:

         (a) Ownership. AT&T owns the Series A Preferred Stock, free and clear
of all liens, claims, charges, encumbrances, and restrictions (other than
restriction imposed by applicable securities laws), and the transfer of the
shares of Series A Preferred Stock to the Company will not constitute a breach
or violation of or default under any agreement or other instrument by which AT&T
may be bound.

         (b) Authority. AT&T has all requisite power and authority to enter into
this Agreement and perform its obligations hereunder, and this Agreement
constitutes a valid and binding obligation of AT&T enforceable against AT&T in
accordance with its terms subject to bankruptcy, insolvency, reorganization, and
similar laws of general application affecting the rights and relief of creditors
and secured parties and to the further extent that the availability

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of the remedies of specific enforcement, of injunctive relief, or of other
equitable remedies is subject to the discretion of the court before which any
proceeding therefor may be brought.

         (c) Accredited Investor. AT&T is an "accredited investor" as that term
is defined in Regulation D under the Securities Act, and is domiciled in the
state of New Jersey.

         (d) State Securities Laws. AT&T is a "financial institution or
institutional buyer" as that term is defined in Section 49:3-50(b)(8) of the New
Jersey Uniform Securities Law.

         5. Representations and Warranties of the Company. As of the Closing,
the Company represents and warrants to AT&T that:

         (a) Corporate Status. Each of the Company and Planet is a corporation
duly organized, validly existing, and in good standing under the laws of the
Commonwealth of Massachusetts and has full corporate power and authority to
carry on its business as now conducted and is registered or qualified to do
business as a foreign corporation in each jurisdiction in which the nature of
the Company's operations and the Company's ownership of property requires the
Company to be so qualified and where the failure so to qualify would have a
material adverse effect upon the business of the Company as currently conducted.
It is intended that following the Closing, Planet will be merged with and into
the Company.

         (b) Capitalization of the Company. The authorized capital stock of the
Company currently consists of 100,000,000 shares of Common Stock, par value
$1.00 per share. At the close of business on July 31, 1996, there were
20,402,365 shares of Common Stock of the Company validly issued and outstanding,
and approximately 6,380,000 shares of Common Stock were subject to authorized
options, convertible securities, and other rights to acquire Common Stock of the
Company. Except for the Common Stock Purchase Rights of the Company and except
as set forth in the preceding sentence, at the close of business on July 31,
1996 there were no commitments, agreements, understandings, or arrangements of
the Company of any kind relating to the issuance of capital stock of the
Company, any securities convertible or exchangeable into capital stock of the
Company, or any such rights, warrants, or options. The shares of Common Stock of
the Company being acquired by AT&T pursuant to this Agreement will, when
delivered to AT&T in accordance with this Agreement, be duly authorized, validly
issued, fully paid, and nonassessable, free and clear of all liens, claims,
charges, encumbrances, and restrictions (other than restrictions imposed by
applicable securities laws).

         (c) SEC Documents. The Company has furnished to AT&T a true and
complete copy of the Company's Form 10-K for the year ended June 30, 1995 and
Form 10-Q's for the periods ended September 30, 1995, December 31, 1995, and
March 31, 1996, and proxy statement for the 1995 annual meeting (the "Company's
SEC Documents"). As of their respective filing dates, the Company's SEC
Documents complied in all material respects with the requirements of the federal
securities laws, and none of the Company's SEC Documents

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contain any untrue statement of a material fact or omit to state a material fact
required to be stated therein or necessary in order to make the statements made
therein, in light of the circumstances under which they were made, not
misleading.

         (d) Authority for Agreement. The Company has all necessary corporate
power and authority to execute and deliver this Agreement and to carry out its
obligations hereunder. The execution and delivery of this Agreement and the
consummation of the transactions contemplated hereby have been duly authorized
by the Board of Directors of the Company and no other corporate action
(including but not limited to stockholder approval) is legally required to be
taken or obtained by the Company to duly authorize the execution, delivery, and
performance of this Agreement and the transactions contemplated hereby. This
Agreement constitutes the valid and legally binding obligation of the Company
and is enforceable against the Company in accordance with its terms subject to
bankruptcy, insolvency, reorganization, and similar laws of general application
affecting the rights and relief of creditors and secured parties and to the
further extent that the availability of the remedies of specific enforcement, of
injunctive relief, or of other equitable remedies is subject to the discretion
of the court before which any proceeding therefor may be brought. The execution
and delivery of this Agreement and the consummation of the transactions
contemplated hereby will not conflict with or result in any material violation
of or material defaults under any provision of the Articles of Organization or
By-laws of the Company or in any violation of or default with respect to any
mortgage, indenture, lease, agreement, or other instrument, permit, concession,
grant, franchise, license, judgment, order, decree, statute, law, ordinance,
rule, or regulation applicable to the Company. Such execution, delivery, and
consummation will not accelerate the maturity of or otherwise modify the
material terms of any indebtedness of the Company, or, result in the creation of
any lien, charge, encumbrance, or security interest upon any of the property or
assets of the Company.

         No consent, approval, order, or authorization of, or registration,
declaration, or filing with, any governmental authority and no consent or other
agreement of any person other than the Company is required in connection with
the execution and delivery of this Agreement or the consummation of the
transactions contemplated hereby by the Company. No consent or other agreement
of any person other than the Company is required for the sale of the Shares by
the Company or for the acceptance by the Company of the Series A Preferred Stock
to be transferred or assigned under this Agreement.

         (e) Brokers, Finders, etc. All negotiations relating to this Agreement
and the transactions contemplated hereby and thereby have been carried on
without the intervention of any person acting on behalf of the Company or in
such manner as to give rise to any valid claim against AT&T for any brokerage or
finder's commission, fee, or similar compensation on behalf of the Company.

         (f) Litigation. To the Company's knowledge, there are no judicial or
administrative actions, suits, proceedings, or investigations pending, or
threatened, which question the

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validity of this Agreement or of any action taken or to be taken pursuant to or
in connection with the provisions of this Agreement, or which could reasonably
be expected to result in any material adverse change in the business or
financial condition of the Company and which have not been publicly disclosed by
the Company.

         (g) Financial Statements. The Company has delivered to AT&T the
consolidated balance sheets of the Company as of June 30, 1995 and 1994 and
related consolidated statements of operations and shareholders' equity and cash
flow for the three years ended June 30, 1995 as stated in a copy of its annual
report to shareholders for the fiscal year ended June 30, 1995 and reported upon
therein by Coopers & Lybrand, independent public accountants. The foregoing
financial statements, including the accompanying notes, have been prepared in
accordance in all material respects with the requirements of the SEC with
respect to preparation of such annual report and fairly present the financial
position of the Company as of the dates of the respective balance sheets and the
results of its operations for the respective periods then ended.

         6. Put Option. The Company hereby grants to AT&T an option (the "Put
Option") to require the Company to purchase all or any part of the Shares then
held by AT&T at a per share price of $20.00 (appropriately adjusted for any
stock splits, reverse stock splits, subdivisions, combinations, and other
reclassifications of the Company's Common Stock), exercisable by AT&T at any
time after the Closing and prior to the 365th day following the Closing (the
"Put Option Exercise Period").

         The Put Option may be exercised by delivery of written notice thereof
(the "Put Notice") to the Company stating that AT&T has elected to exercise the
Put Option and the number of Shares with respect to which the Put Option is
being exercised.

         The closing of the purchase of Shares pursuant to an exercise of the
Put Option shall take place 20 business days from the date the Put Notice was
given, at 10:00 a.m. (local time) at the principal office of the Company, or at
such other time and location as the parties may mutually determine. At the
closing the Company shall pay to AT&T the aggregate purchase price for the
Shares to be purchased pursuant to the Put Option by certified or bank check. At
such time, AT&T shall deliver to the Company the certificate or certificates
representing the Shares so repurchased, duly endorsed for transfer and with
signature guaranteed, free and clear of any liens or encumbrances (other than
restriction imposed by applicable securities laws) with any necessary stock
transfer tax stamps affixed.

         Upon and following the first closing of the purchase of Shares pursuant
to the exercise of the Put Option, all rights of Holder to Demand Registration
of Registrable Securities under Section 2(a) of this Agreement shall cease and
terminate. Notwithstanding any termination of Demand Registration rights under
the provisions of the preceding sentence, the rights of Holder to Piggyback
Registration of Registrable Securities shall continue pursuant to the terms of
Section 2 of this Agreement.

                                      -11-

 
   12
        AT&T's rights under this Section 6 may be assigned or transferred to
the same extent, but together with and not separate from, as the rights to cause
the Company to register securities may be transferred pursuant to Section 2(n).

         7.  Miscellaneous.

         7.1. Expenses. The Company agrees to reimburse AT&T for 50% of the
aggregate amount of outside fees and disbursements of AT&T's legal and financial
advisors in connection with this Agreement; provided, however, that the
aggregate payment(s) by the Company pursuant to this Section 7.1 may in no event
exceed $60,000.

         7.2. Notices. All notices, requests, consents, and demands shall be in
writing and shall be personally delivered, mailed, postage prepaid; or
telecopied, to the Company at:

                           BBN Corporation
                           150 CambridgePark Drive
                           Cambridge, Massachusetts 02140
                           Attn:  President

to Holder at:

                           295 North Maple Avenue
                           Basking Ridge, N.J. 07920
                           Attn:    Dominick Turiano
                                    Chief Financial Officer
                                    Room 3360A2
                                    Tel: 908-221-6610
                                    Fax: 908-630-1455

to Planet at:

                           150 CambridgePark Drive
                           Cambridge, MA 02140
                           Attn:  President

or such other address as may be furnished in writing to the other parties
hereto.

         All such notices, requests, demands, and other communication shall,
when mailed (registered or certified mail, return receipt requested, postage
prepaid) be effective four days after deposit in the mails, or when personally
delivered or when telecopied, shall be effective upon actual receipt.

                                      -12-

 
   13
         7.3. Entire Agreement. This Agreement constitutes the entire agreement
of the parties with respect to the matters contemplated herein. It supersedes
any and all prior understandings or agreements as to the subject matter of this
Agreement. Upon the effectiveness of this Agreement, the Series A Preferred
Stock Purchase Agreement dated as of July 5, 1995 between Planet and AT&T, and
the related Co-Sale Agreement dated as of July 5, 1995 among the parties hereto,
are each canceled and terminated in whole, without liability or claims to any
party thereto.

         7.4. Amendments, Waivers, and Consents. Any provision in this Agreement
to the contrary notwithstanding, changes in or additions to this Agreement may
be made, and compliance with any covenant or provision herein set forth may be
omitted or waived, upon the consent thereto in writing of the Company and
Holder.

         7.5. Binding Effect; Assignment. This Agreement shall be binding upon
and inure to the benefit of the personal representatives, successors, and
permitted assigns, if any, of the respective parties hereto. Holder shall not
have the right to assign its rights or obligations hereunder or any interest
therein without the written consent of the Company or except as provided in
Section 2(n) and the last paragraph of Section 6. The Company shall not have the
right to assign its obligations hereunder or any interest therein without
obtaining the prior written consent of the Holder.

         7.6. General. The headings contained in this Agreement are for
reference purposes only and shall not in any way affect the meaning or
interpretation of this Agreement. In this Agreement the singular includes the
plural, the plural, the singular, the masculine gender includes the neuter,
masculine and feminine genders.

         7.7. Severability. If any provisions of this Agreement shall be found
by any court of competent jurisdiction to be invalid or unenforceable, the
parties hereby waive such provision to the extent that it is found to be invalid
or unenforceable. Such provision shall, to the maximum extent allowable by law,
be modified by such court so that it becomes enforceable, and, as modified,
shall be enforced as any other provision hereof, all the other provisions hereof
continuing in full force and effect.

         7.8. Counterparts. This Agreement may be executed in counterparts, all
of which together shall constitute one and the same instrument.

         7.9. Specific Performance. The Company recognizes that the rights of
Holder under this Agreement are unique, and, accordingly, Holder shall, in
addition to such other remedies as may be available to it at law or in equity,
have the right to enforce its rights hereunder by actions for injunctive relief
and specific performance to the extent permitted by law. This Agreement is not
intended to limit or abridge any rights of Holder which may exist apart from
this Agreement.

         7.10. Other.

                                      -13-

 
   14
        (a) The Company will cooperate with all reasonable requests by Holder
for such publicly-available information relating to the Company as may be called
for by an information- reporting form currently or hereafter required by the SEC
as a condition to the availability of an exemption from the Securities Act for
the sale of any Restricted Securities, including any publicly-available
information relating to the Company that may be required to be delivered by
Holder in connection with any sales to accredited investors (as defined in
Regulation D promulgated under the Securities Act or any successor regulation)
in a resale pursuant to the so-called "4(1 1/2) exemption" or to any qualified
institutional buyer in a resale pursuant to Rule 144A promulgated under the
Securities Act or any successor regulation. In this connection, the Company will
furnish to Holder, promptly upon request by Holder, copies of all financial
statements, reports, notices and proxy statements, sent or made available
generally by the Company to its stockholders, and copies of all regular and
periodic reports and all registration statements and prospectuses filed by the
Company with any securities exchange or with the SEC.

         (b) Until the date on which Holder no longer owns any Shares, the
Company will cause to be delivered promptly to Holder copies of annual and
quarterly reports and all other final documents, reports, and information
publicly filed by the Company with the Securities and Exchange Commission
pursuant to the Securities Act or the Exchange Act. In addition, until such date
the Company shall (a) fax to Holder, at the fax number provided in or pursuant
to Section 7.2, copies of all Company press releases and other announcements
promptly following the time such releases or announcements are publicly made,
and (b) shall provide to Holder an opportunity to participate in all
conferences, whether by telephone or in person, generally made with security
analysts, including any such conferences held in respect of quarterly earnings
announcements (in respect of any of which the Company shall provide Holder with
reasonable notice in advance of such conference).

              [THE REMAINDER OF THIS PAGE INTENTIONALLY LEFT BLANK]

                                      -14-

 
   15
         IN WITNESS WHEREOF, the parties have caused this Exchange and
Registration Rights Agreement to be duly executed as of the date first above
written.

                                          BBN CORPORATION

                                          By  /s/ John Montjoy
                                              -----------------------------
                                              Title: Senior V.P.

                                          BBN PLANET CORPORATION

                                          By  /s/ R. Goldwasser
                                             ------------------------------
                                              Title: V.P.

                                          AT&T VENTURE COMPANY, L.P.

                                          By:  Venture Management I, G.P.

                                               By /s/ William H. Eliot
                                                  -------------------------
                                                      William H. Elliott
                                                      General Partner

                                      -15-